Method of Production of Hot Dipped Hot Rolled Steel Strip

ABSTRACT

Production of hot dipped hot rolled steel strip by producing steel strip by casting by a thin slab continuous casting process and hot rolling, said steel strip containing, by mass %, C: 0.03% or more, Si: 0.02% or more, Mn: 0.15% or more, and Ca: 0.001% or more by heating it to a peak maximum steel strip temperature of 550° C. to less than 650° C. by a temperature elevation rate of 25° C./sec or more for 15 sec or more for oxidation, heating it to a peak maximum steel strip temperature of 700° C. to 760° C. so that the time when the steel strip temperature is 570° C. or more is 25 sec to 45 sec for reduction, then hot dipping it.

TECHNICAL FIELD

The present invention relates to a method of production of hot dippedplating hot rolled steel strip produced by the thin slab continuouscasting process, hot rolling process and hot dipping plating process.

BACKGROUND ART

In recent years, due to the need to save energy and cut costs,technology for production of steel plate using the thin slab continuouscasting process (thin slab casting process) such as described inJapanese Patent Publication (A) No. 2-197358 has come under thespotlight in the world. This thin slab continuous casting process ischaracterized by the point of the thin strip being directly sent fromthe continuous casting process to the rolling process. For this reason,compared with a conventional continuous casting machine requiringcooling of the steel slab, fault detection, fault removal, heating, andnumerous other processes between the continuous casting process androlling process, the energy efficiency is extremely good and the capitalcosts can be kept low. Further, the fact that this thin slab continuouscasting machine can be utilized together with electric furnaces usingscrap as raw materials is another major reason why attention is beinggathered.

However, there is the problem that hot rolled steel strip produced bythe thin slab continuous casting process is harder to improve in surfacequality compared with hot rolled steel strip produced by a conventionalcontinuous casting machine. For this reason, up until recently, the thinslab continuous casting process has not spread in use that widely.Further, there is very little information on hot rolled steel stripproduced by the thin slab continuous casting process. When hot dipgalvanizing this hot rolled steel strip, the method used for hot rolledsteel strip obtained by a conventional continuous casting machine hasbeen used as it is.

As the method for hot dip galvanization of hot rolled steel strip, ingeneral a “non-oxidizing furnace method” is used. With this method, hotrolled steel strip is continuously run through a non-oxidizing furnace,reduction furnace (annealing furnace), and cooling furnace to heat it toa high temperature and oxidize and reduce it. By oxidizing hot rolledsteel strip in the non-oxidizing furnace, then reducing it in thereduction furnace in this way, an Fe layer can be formed on the hotrolled steel strip surface. The FeO or other oxide film on the hotrolled steel strip surface is resistant to adherence by the hot melt, soremoving this from the surface of the hot rolled steel strip has theeffect of improving the plating wettability for hot dipping.

Such a conventional hot dipping facility is designed mainly for thepurpose of processing cold rolled steel sheet, so the temperatureelevation rate in the heating zone was about 10° C./s to 20° C./s inrange. Further, when using this hot dipping facility to plate hot rolledsteel strip, since general steel does not require recrystallizationannealing, the maximum temperature at the time of annealing was usuallyadjusted to 640° C. to 660° C. or so.

Note that, as another method, the “flux method” is also known. With thismethod, the hot rolled steel strip surface is coated with a flux of zincchloride, ammonium chloride, etc. to activate the hot rolled steel stripsurface and improve the wettability for the hot dipping. However, thismethod is not generally used for the production of hot dipped steelstrip in view of the difficulty of continuous production and platingadhesion.

If the hot rolled steel strip produced using the thin slab continuouscasting process is hot dip galvanized by the method of production of hotdipped steel strip using the above-mentioned “non-oxidizing furnace typeplating facility”, nonplating defects are formed on the surface of thehot dip galvanized steel strip. This is believed to be partially due tothe addition of Ca specific to the thin slab continuous casting process.

A thin slab continuous casting machine has a much narrower casting moldthan a conventional continuous casting machine and has an injectionnozzle of a special structure as well, so alumina easily clogs thenozzle. Therefore, to prevent this, in a thin slab continuous castingmachine, Ca is added to the ladle to lower the melting point of thealumina.

In the thin slab continuous casting process, a cast 50 mm to 80 mm or sothickness slab is sent directly to the rolling process while held at ahigh temperature and rolled. This hot rolling mill is a hot rolling millcorresponding to a final rolling machine of a conventional hot rollingprocess and rolls a slab to a thickness of 1.2 mm to 4 mm or so toproduce hot rolled steel strip. In this case, to keep the thin slabwarm, a tunnel furnace with a long residence time is used, so a largeamount of scale is formed on the slab surface before rolling.

The Ca added as explained above and remaining in the thin slab oxidizesin the scale and remains in the form of CaO. As a result, the oxide CaOformed by this addition of Ca causes unevenness and pitting in the oxidefilm on the surface of the hot rolled steel strip when oxidized in thenon-oxidizing furnace in the plating process, causes partial degradationof the plating wettability with the hot dip galvanization, and causesplating defects.

Further, the hot rolled steel strip produced using the thin slabcontinuous casting process exhibits a greater amount of smut comparedwith a conventional continuous casting machine. This is because with thethin slab continuous casting process, the cast steel thin slab isdirectly sent to the rolling process and rolled while keeping it at ahigh temperature, so Fe₃C and C easily remain on the steel strip surfacein the separated state. If a lot of these Fe₃C etc. remain on thesurface of the hot rolled steel strip, when oxidized in thenon-oxidizing furnace, the C reacts with the oxygen, the formation of anFe oxide film is partially delayed, and unevenness and pitting areformed on the oxide film. These unevenness and pitting are considered tolower the plating wettability with zinc and cause plating defects.

Further, it was learned that if the hot rolled steel strip producedusing the thin slab continuous casting process is produced by aconventional hot dipping line, coil breakage will occur. In particular,remarkable coil breakage similar to “fluting” occurs with hot rolledsteel strip of a thickness of 2 mm or more. The reason is that ifproduced by a conventional hot dipping line, the yield point falls morethan necessary at the heating and annealing stage, so in particular ifprocessing thick-gauge hot rolled steel strip of a thickness of 2 mm ormore, coil breakage occurs on the processing line after the plating.

To prevent coil breakage, in the past the technology of heating the hotrolled steel strip after plating to adjust the yield point and thetechnology of increasing the roll diameter of the processing line afterplating to reduce the bending strain have been proposed, but the formertechnology is complicated in operation. The latter technology requiresprecision processing of the roll profile etc. to produce large diameterrolls and therefore sophisticated technology and processing facilities,so as a result requires considerably high cost for production of therolls.

DISCLOSURE OF THE INVENTION

The present invention was made in consideration of the above problemsand in particular provides a means for preventing nonplating defectsformed on a plate surface when hot dipping hot rolled steel stripproduced by the thin slab continuous casting process.

To solve the above problems, according to the present invention, thereis provided a method of production of hot dipped hot rolled steel stripcharacterized by producing steel strip by casting by a thin slabcontinuous casting process and hot rolling, said steel strip containing,by mass %, C: 0.03% or more, Si: 0.02% or more, Mn: 0.15% or more, andCa: 0.001% or more, heating it to a peak maximum steel strip temperatureof 550° C. to less than 650° C. by a temperature elevation rate of 25°C./sec or more for 15 sec or more for oxidation, heating it to a peakmaximum steel strip temperature of 700° C. to 760° C. so that the timewhen the steel strip temperature is 570° C. or more is 25 sec to 45 secfor reduction, then hot dipping it.

Note that, in the method of production of hot dipped hot rolled steelstrip, the hot dipping may be made hot dip galvanization.

Further, according to the present invention, there is provided afacility for production of hot dipped hot rolled steel strip which hotdips steel strip produced by casting by the thin slab continuous castingprocess and by hot rolling, which facility for production of hot dippedhot rolled steel strip is characterized by having a furnace used foroxidation and a furnace used for reduction and by a ratio of lengthbetween the furnace used for oxidation and the furnace used forreduction along a conveyance direction of the hot rolled steel stripbeing 0.5 to 0.9.

Note that, in the facility for production of hot dipping hot rolledsteel strip, the steel strip can pass through the furnace used foroxidation in a time of 15 sec to 25 sec.

According to the present invention, it is possible to prevent nonplatingdefects formed on the plated surface when hot dipping hot rolled steelstrip produced by the thin slab continuous casting process. Further, itis also possible to perform the hot dipping without coil breakage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view of the configuration of a suitable hot dip galvanizedhot rolled steel strip production facility according to the presentinvention.

FIG. 2 is a view explaining the temperature changes at a non-oxidizingfurnace and annealing furnace of a suitable hot dip galvanized hotrolled steel strip production facility according to the presentinvention.

FIG. 3 gives views before and after oxidation of hot rolled steel stripproduced by the thin slab continuous casting process. (a) shows hotrolled steel strip before oxidation, (b) shows hot rolled steel stripafter oxidation by the present invention, and (c) shows hot rolled steelstrip after oxidation by the prior art.

FIG. 4 gives views of the hot rolled steel strip oxidized in anon-oxidizing furnace before and after reduction. (d) shows hot rolledsteel strip before reduction, (e) shows hot rolled steel strip reducedwithout excess or shortage, (f) shows hot rolled steel strip which isinsufficiently reduced, and (g) shows hot rolled steel strip which isexcessively reduced.

FIG. 5 is a view of the configuration of a washing apparatus in front ofthe hot dipping apparatus.

BEST MODE FOR CARRYING OUT THE INVENTION

Below, preferred embodiments of the present invention will be explainedwhile referring to the drawings. Note that in this Description and thedrawings, elements having substantially the same functionalconfigurations are assigned the same reference numerals.

In the present invention, as the hot dipping steel strip produced by themethod of production of hot dip galvanized hot rolled steel strip, hotdip galvanized steel strip SGHC, SGH340, SGH400, SGH440, SGH540, etc.defined by JIS G 3302 are covered. Hot rolled steel strip produced bycasting and rolling steel containing, by mass %, C: 0.03% or more, Si:0.02% or more, Mn: 0.15% or more, and Ca: 0.001% or more by the thinslab continuous casting process is used.

If Ca is less than 0.001%, nozzle clogging sometimes cannot beprevented, so at least that amount is contained. Ca is usually added inthe steelmaking process by adding CaAl, CaSi, FeCa, or metallic Ca tothe molten steel after deoxidation.

FIG. 1 is a view of the configuration of a suitable facility forproduction 1 of hot dip galvanized hot rolled steel strip according tothe present invention. This facility for production of hot dipgalvanized hot rolled steel strip is comprised of a feeding reel 10serving as the starting point of the hot dip galvanization process line,a coiling reel 11 serving as the end point, a preheating furnace (notshown) arranged between the reels 10, 11, a non-oxidizing furnace 12, anannealing furnace 15 including a reduction zone 13 and cooling zone 14,a hot dip galvanization tank 16, a wiping apparatus 17, and a coolingfurnace 18.

The feeding reel 10 is a reel on which is coiled hot rolled steel stripproduced by casting steel containing, by mass %, C: 0.03% or more, Si:0.02% or more, Mn: 0.15% or more, and Ca: 0.001% or more by the thinslab continuous casting process, then rolling it as is without loweringthe temperature.

The non-oxidizing furnace 12 is known as a furnace for “slight”oxidation of the hot rolled steel strip fed out from the feeding reeland has a length in the conveyance direction of the steel strip of forexample 15 m to 25 m. In the case of this embodiment, the processingrate is 120 m/min, so the oxidation time of the hot rolled steel stripin the non-oxidizing furnace 12 is 7 sec to 12 sec. The fuel-air ratioin the non-oxidizing furnace 12 is set to 0.9 to 0.98 or so. Further,the length in the conveyance direction of the non-oxidizing furnace 12plus the preheating furnace is set to for example 30 m to 50 m. Theoverall oxidation time (passage time) in the non-oxidizing furnace 12and preheating furnace becomes 15 sec to 25 sec.

The annealing furnace 15 arranged right after the non-oxidizing furnace12 is comprised of the reduction zone 13 for reducing the oxidized hotrolled steel strip and the cooling zone 14 for cooling the hot rolledsteel strip and has a length in the conveyance direction of for example70 m to 100 m. In the case of this embodiment, the processing rate is120 m/min, so the reduction time of the hot rolled steel strip in theannealing furnace 15 becomes for example 25 sec to 45 sec in the regionof 570° C. or more where the reduction is relatively fast. Further, H₂and N₂ etc. are made the atmosphere in the annealing furnace 15. Notethat the reduction zone in which reduction is mainly performed iscomprised of a reduction furnace and soaking furnace or just a reductionfurnace. Its length in the conveyance direction is for example set to 50m to 70 m.

The hot dip galvanization tank 16 is a tank for treating hot rolledsteel strip for deposition by hot dipping. The wiping apparatus 17 is anapparatus for wiping off excessive molten melt adhered to the hot rolledsteel strip by a gas. The cooling furnace 18 is a furnace for thencooling the hot rolled steel strip.

Next, the method of production of hot dip galvanized hot rolled steelstrip using the hot dip galvanized hot rolled steel strip productionfacility 1 will be explained while using FIG. 2 to FIG. 4.

FIG. 2 is a view showing the change in temperature of the steel stripsurface when the hot rolled steel strip passes through the non-oxidizingfurnace 12, the reduction zone 13, and the cooling zone 14 of thefacility for production 1 of hot dip galvanized hot rolled steel strip.In FIG. 2, the temperature point where the hot rolled steel strip entersthe non-oxidizing furnace 12 is O, the temperature point where it leavesthe non-oxidizing furnace 12 is P, the temperature point where it entersthe reduction furnace of the reduction zone 13 is Q, the temperaturepoint where it leaves the reduction furnace of the reduction zone 13 andenters the soaking furnace of the reduction zone 13 is S, thetemperature point where it leaves the soaking furnace of the reductionzone 13 and enters the cooling zone 14 is T, and the temperature pointwhere it leaves the cooling zone 14 is V.

First, the hot rolled steel strip produced by the thin slab continuouscasting process is fed out from the feeding reel 10, proceeds on theline, passes through the preheating furnace, and enters thenon-oxidizing furnace 12.

The hot rolled steel strip entering the non-oxidizing furnace 12, asshown by the section I in FIG. 2, is heated so that the peak maximumsteel strip temperature becomes 550° C. to less than 600° C. at atemperature elevation rate of 25° C./sec or more for a period of 15 secto 25 sec, whereby the surface of the hot rolled steel strip isoxidized. Here, the “oxidation time” means the time of passage throughthe preheating zone and non-oxidizing furnace.

The states of the hot rolled steel strip surface before and after thisoxidation are shown in FIG. 3. FIG. 3( a) shows the hot rolled steelstrip before oxidation, FIG. 3( b) shows the hot rolled steel stripafter oxidation by the present invention, and FIG. 3( c) shows the hotrolled steel strip after oxidation by the prior art.

By setting the temperature elevation rate in the section I of FIG. 2 to25° C./sec or more, which is higher than the above-mentionedconventional temperature elevation rate, the effect of preventing theformation of nonplating defects is obtained. As opposed to this, ifsetting the temperature elevation rate in the section I at less than 25°C./sec, the oxide CaO and calcium aluminate formed by the addition of Caand the smut Fe₃C etc. cause the formation of nonplating defects. Thereason why setting the temperature elevation rate at 25° C./sec or moreprevents nonplating defects will be explained below.

As shown in FIG. 3( a), Fe oxide film on the hot rolled steel stripsurface is formed by the Fe atoms of the Fe layer moving to the surfacelayer and reacting with oxygen. Further, when an Fe oxide film isformed, the Si and Mn present in the steel strip are oxidized in thesame way as Fe, so SiO₂ and MnO and other secondary oxide films areformed under the Fe oxide film. Here, when an Fe oxide film is formed,if the CaO, Fe₃C, etc. shown in FIG. 3( a) adhere to the steel stripsurface, the formation of an Fe oxide film will be inhibited and thepits 19 shown in FIG. 3( c) will end up being formed. In the case ofFe₃C, this breaks down into C which reacts with oxygen whereby, as shownin FIG. 3( c), the formation of an Fe oxide film is inhibited. In theabove way, if pits 19 are formed, as shown in FIG. 3( c), SiO₂ and MnOand other secondary oxide films end up appearing on the surface. TheseSiO₂ and MnO and other secondary oxide films degrade the wettabilitywith the hot dip galvanization, so nonplating defects end up beingcaused at the time of hot dip galvanization.

Therefore, in the present invention, the temperature elevation rate isset to a high value of 25° C./sec or more and the rate of formation ofan Fe oxide film is made large.

If the heating temperature becomes high, formation of an oxide film willbe promoted, so the larger the heating rate, the greater the rate offormation of an oxide film. An oxide film is formed mainly by movementto the Fe surface, so if the rate of formation of an oxide film islarge, in the end CaO, Fe₃C, etc. will be pushed out at the steel stripsurface. Even if CaO, Fe₃C, etc. result in pits, Fe oxide film will alsobe formed at their bottom.

The action is believed to occur since the concentration of oxygen at thesteel strip surface is high at the time of heating, so Fe₂O₃ (hematite)is formed at the extreme surface of the hot rolled steel strip. Theformation of Fe₂O₃ is said to proceed due to the diffusion of oxygen tothe inside of the steel strip. From this, it may be considered that as aresult CaO, Fe₃C, etc. are pushed out at the steel strip surface.

The concentration of oxygen inside the Fe oxide film of the surfacebecomes smaller the more to the inside from the surface, so under theFe₂O₃, Fe₃O₄ (magnetite) is formed at 570° C. or less while FeO(wüstite) is formed at 570° C. or more. These Fe₃O₄ and FeO grow due tooutward diffusion of Fe ions. Therefore, at 570° C. or more, Fe₂O₃ isformed at the extreme surface of the hot rolled steel strip, Fe₃O₄ isformed below that, and FeO is formed below that. At less than 570° C.,Fe₂O₃ is formed at the extreme surface and Fe₃O₄ is formed below that.

Below these FeO and Fe₂O₃, when the concentration of Si or Mn in thesteel is high, secondary oxide films comprised of Si or Mn oxides or Siand Mn composite oxides are formed.

If CaO, Fe₃C, etc. adhere to the hot rolled steel strip surface and arenot pushed out at the surface, the CaO, Fe₃C, etc. will block the supplyof oxygen from the surface layer, so secondary oxide films comprised ofSi or Mn oxides or Si and Mn composite oxides will be directly formedbelow the CaO, Fe₃C, etc. In this case, in the succeeding reductionprocess, if the surface CaO, Fe₃C, etc. drop off, pits of Si or Mnoxides or Si and Mn composite oxides exposed at the surface will beformed and as a result nonplating defects will be detected afterplating.

However, as explained above, when setting the temperature elevation rateto a high value of 25° C./sec or more, the CaO, Fe₃C, etc. adhered tothe steel strip surface are pushed out at the surface, the concentrationof oxygen at the pits after they are pushed out becomes high, and Fe₃O₄and FeO are formed at these parts, so Si or Mn oxides or Si and Mncomposite oxides will never be exposed at the surface.

Due to this, even if the pits 19 shown in FIG. 3( b) are formed at theFe oxide film due to the inhibitory actions of CaO, Fe₃C, etc., an Feoxide film is formed at the bottom of this pitting 19. Therefore, theSiO₂, MnO, and other secondary oxide films are covered by the Fe oxidefilm and will not appear at the steel strip surface.

That is, the properties of the steel strip surface after the end of thetemperature elevation process become as follows: As shown in FIG. 3( b),from the inside, the surface is comprised of Fe (hot rolled steelstrip), a secondary oxide film comprised of Si or Mn oxides or Si and Mncomposite oxides, and an oxide film comprised of Fe₃O₄ and FeO or FeOover that. CaO, Fe₃C are present at the surface. There are pits underthe CaO, Fe₃C, but there is the FeO layer.

As opposed to this, if setting the temperature elevation rate at lessthan 25° C./sec, CaO, Fe₃C, etc. will be hard to push out at thesurface, so as shown in FIG. 3( c), a secondary oxide film comprised ofSi or Mn oxides or Si and Mn composite oxides will end up appearing atthe surface.

Note that, the secondary oxide films comprised of Si or Mn oxides or Siand Mn composite oxides on Fe (hot rolled steel strip) will be simplydescribed in FIGS. 3( b), (c) as “SiO₂, MnO”.

Further, by setting the peak maximum steel strip temperature in thenon-oxidizing furnace to 550° C. or more, the effect is obtained that anoxide layer is uniformly formed and the CaO, Fe₃C, etc. present at thesurface part of the oxide film can be easily removed. This effect is notobtained if the peak maximum steel strip temperature is made less than550° C.

Further, by setting the peak maximum steel strip temperature in thenon-oxidizing furnace to less than 600° C., excessive formation of anoxide film is prevented. If the peak maximum steel strip temperatureinside the non-oxidizing furnace is made 600° C. or more, the oxide filmwill be excessively produced and oxide film will end up remaining in thesubsequent reduction.

In this case, the time for holding the temperature elevation rate at 25°C./sec or more is made 15 sec or more. If less than 15 sec, a sufficientoxide film thickness is not possessed, so as a result, the secondaryoxide films comprised of Si or Mn oxides or Si and Mn composite oxideswill end up being exposed at the surface without being covered by theFeO film.

Next, as shown by the section II of FIG. 2, the oxidized hot rolledsteel strip proceeds on the line and enters the reduction zone 13 of theannealing furnace 15. In the annealing furnace 15, first, the strip isheated in the reduction zone 13 to give a peak maximum steel striptemperature of 700° C. to 760° C., then proceeds to the cooling zone 14where it is cooled. The hot rolled steel strip is reduced in thereduction zone 13 and the cooling zone 14 in the annealing furnace in astate holding the steel strip temperature at 570° C. or more for aperiod of 25 sec to 45 sec. That is, in FIG. 2, the time from thetemperature point R where the steel strip temperature is 570° C. to thetemperature point U is set to 25 sec to 45 sec.

Here, the reason for limiting the temperature of the reduction to theregion of a temperature of 570° C. or more is as follows: That is, above570° C., FeO becomes the main Fe oxide and is reduced, while at lessthan 570° C., Fe₃O₄ becomes the main Fe oxide and is reduced. FeO,compared with Fe₃O₄, is easier to reduce due in part to the highprocessing temperature. Therefore, the method of reducing FeO is easierto control than reduction of Fe₃O₄.

The hot rolled steel strip surfaces before and after the above reductionare shown in FIG. 4. The hot rolled steel strip before reduction is (d),the hot rolled steel strip reduced without excess or shortage is (e),the hot rolled steel strip which is insufficiently reduced is (f), andthe hot rolled steel strip which is excessively reduced is (g). Notethat, in FIG. 4, the CaO and Fe₃C shown in FIG. 3 are not shown, butthese CaO and Fe₃C are blown away from the steel strip surface by theflow or the reduction atmosphere H₂, N₂, and the like when passingthrough the annealing furnace 13 etc.

Note that, the secondary oxide films comprised of Si or Mn oxides or Siand Mn composite oxides formed on the Fe (steel strip) are describedsimply as “SiO₂, MnO” in FIG. 4 as well.

As a result, the oxide film in the state of FIG. 3( b) is suitablyreduced and, as shown in FIG. 4( e), the structure becomes, from theinside, Fe (steel strip), a secondary oxide film comprised of Si or Mnoxides or Si and Mn composite oxides, and a film of Fe above that. Pitswhere CaO and Fe₃C had been present remain on the surface, but there isan Fe layer at the bottom.

By reducing the hot rolled steel strip to give a peak maximum steelstrip temperature of 700° C. to 760° C. in a state holding the steelstrip temperature at 570° C. or more for a period of 25 sec to 45 sec,the surface of the hot rolled steel strip shown in FIG. 4( d) is reducedwithout excess or shortage in the annealing furnace 15.

That is, as shown in FIG. 4( e), the Fe oxide film formed by thenon-oxide film is reduced and becomes a completely Fe layer. Further,this Fe layer completely covers the SiO₂, MnO, and other secondary oxidefilms formed by the oxidation and reduction as well. The SiO₂, MnO, andother secondary oxide films degrading the plating wettability with thehot dip galvanization are completely covered, so the plating wettabilitybecomes extremely good, and nonplating defects do not occur.

As opposed to this, when the peak maximum steel strip temperature isless than 700° C. or when the time for holding the steel striptemperature at 570° C. or more is less than 25 sec, the reduction at theannealing furnace 15 becomes insufficient and, as shown in FIG. 4( f),Fe oxide film ends up remaining. Therefore, this Fe oxide film degradesthe plating wettability for hot dipping, so nonplating defects end upoccurring.

Further, when the peak maximum steel strip temperature exceeds 760° C.or the time for holding the steel strip temperature at 570° C. or moreexceeds 45 sec, the reduction in the annealing furnace 15 becomesexcessive. In this case, as shown in FIG. 4( g), the Fe oxide film issufficiently reduced and an Fe layer is formed. However, Si and Mn havea stronger oxidizing power than Fe, so even when the Fe oxide film isreduced by the annealing furnace 15, secondary oxide layers of SiO₂ andMnO excessively grow and end up appearing at the steel strip surface. Asexplained above, SiO₂ and MnO degrade the plating wettability of hotrolled steel strip, so nonplating defects end up being formed.

Next, the reduced hot rolled steel strip proceeds on the line from theannealing furnace 15 to a hot dip galvanization tank 16 heated to apredetermined temperature where it is dipped and hot dip galvanized.

Next, the hot dip galvanized hot rolled steel strip proceeds on the lineand the deposition of the hot dip galvanization on the hot rolled steelstrip is adjusted to a predetermined amount by a wiping apparatus 17.

Next, the hot rolled steel strip proceeds on the line and is cooled inthe cooling furnace 18.

In the above embodiment, the hot rolled steel strip entering thenon-oxidizing furnace 12 is heated to give a peak maximum steel striptemperature of 550° C. to less than 600° C. at a temperature elevationrate of 25° C./sec or more over a period of 15 sec to 25 sec foroxidation. When an Fe oxide film is formed, even if the Fe₃C and othersmut and Ca-based oxides form pits 19, the bottom of the pits 19 arecovered by the Fe oxide film.

Further, in the above embodiment, the oxidized hot rolled steel strip isheated to give a peak maximum steel strip temperature of 700° C. to 760°C. while holding the steel strip temperature at 570° C. or more for 25sec to 45 sec to reduce it, whereby the Fe oxide film on the hot rolledsteel strip surface is reduced without excess or shortage. Further, nosecondary oxide layers of SiO₂ and MnO appear on the surface either.Therefore, the formation of nonplating defects can be prevented.

Further, in the above embodiment, the length in the conveyance directionof the furnace used for oxidation (preheating furnace and non-oxidizingfurnace 12) was set to 30 m to 50 m, while the length in the conveyancedirection of the furnace used for reduction (reduction zone 13) was setto 50 m to 70 m. From experiments, it reveals that if the ratio oflengths along the conveyance direction of the furnace used for oxidationand the furnace used for reduction is 0.5 to 0.9, a good plating statecan be obtained. In the present embodiment, by setting the ratio oflengths along the conveyance direction of the furnace used for oxidationand the furnace used for reduction to be 0.5 to 0.9, the formation ofnonplating defects can be prevented. Further, the furnace used foroxidation and the furnace used for reduction are set to suitable lengthswithout excess or shortage, so the investment in capital cost areoptimized.

Above, a preferred embodiment of the present invention was explainedwhile referring to the attached drawings, but the present invention isnot limited to these examples. A person skilled in the art could clearlyconceive of various modifications or changes within the scope of thetechnical ideas described in the claims. These should naturally also beunderstood as falling under the technical scope of the presentinvention.

Further, in the above embodiment, the hot rolled steel strip was fed outfrom a feeding reel, but it is also possible to directly connect it to aline performing thin slab continuous casting.

Further, in the above embodiment, the hot rolled steel strip was fed outfrom a feeding reel to the non-oxidizing furnace, but it may also betreated by pickling, surface scrubbing, etc. before being fed out to thenon-oxidizing furnace.

Further, in the above embodiment, the hot rolled steel strip was fed outfrom a feeding reel to the inside of the non-oxidizing furnace, but itis also possible to provide an apparatus for pickling, surfacescrubbing, and other processing before oxidation.

Further, in the above embodiment, an annealing furnace including areduction zone and cooling zone was used, but it is also possible to useseparate furnaces such as a reduction furnace and a cooling furnace.

Further, in the above embodiment, as the hot dipping, hot dipgalvanization was used, but aluminum, lead, tin, etc. may also be usedother than zinc.

Further, in the above embodiment, the present invention is particularlyeffective in hot rolled steel strip. The reason is believed to be thatthe surface of hot rolled steel strip has coarser grain boundaries,larger surface areas, easier oxidation and reduction, and larger growthrate of the oxide layer.

Here, to compare the amount of oxidation and amount of reduction underthe hot dip galvanization conditions of cold rolled steel sheet, theconventional formulas for estimating the amount of oxidation and amountof reduction of cold rolled steel sheet are applied to hot rolled steelstrip giving a good plating state under the oxidation and reductionconditions of the present invention so as to calculate the amount ofoxidation and amount of reduction of hot rolled steel strip.

The formula for estimating the amount of oxidation of cold rolled steelsheet estimates the amount of oxidation from the two variables of timestayed in the preheating furnace and non-oxidizing furnace and the peaktemperature of the cold rolled steel sheet. The formula for estimatingthe amount of reduction of cold rolled steel sheet estimates the amountof reduction from the two variables of time stayed in the reductionfurnace and the peak temperature of the cold rolled steel sheet. Whenestimating this amount of reduction, the amount of reduction in the caseof a temperature of the reduction furnace of 570° C. or more and theamount of reduction in the case of less than 570° C. are separatelycalculated and the sum of the two is estimated as the amount ofreduction. The specific forms of the formulas for estimation of theamount of oxidation and amount of reduction are not shown, but can bederived from experiments.

Hot rolled steel strips obtained by hot rolling cast slabs obtained by athin slab casting machine were oxidized and reduced under suitableoxidation and reduction conditions defined by the present invention. Thevalues of the amounts of oxidation and amounts of reduction were foundby the above formulas for estimating the amount of oxidation and amountof reduction. As a result, the amounts of oxidation were 0.12 to 0.2mg/m² or so, and the amounts of reduction were 0.2 to 0.35 mg/m² or so.These values are smaller compared with the amounts of oxidation of 0.1to 0.8 mg/m² and amounts of reduction of 0.45 to 1 mg/m² of cold rolledsteel sheet obtained by the same formulas.

From the above results, the oxidation rate and the reduction rate arefaster than the case of cold rolled steel sheet, so it can be estimatedthat the calculated values of the suitable amount of oxidation andamount of reduction when hot dip galvanizing hot rolled steel stripwould give smaller values than the values in the case of cold rolledsteel sheet.

By applying the present invention to hot dip galvanization of hot rolledsteel strip, compared with the case of application to cold rolled steelsheet, the oxidation time and reduction time can be shortened. Further,the length of the furnaces for the oxidation and reduction can beshortened and therefore the hot dip galvanization facility can bereduced in size.

However, in front of the hot dipping facility of the present invention,as shown in FIG. 5, an alkali washing system comprised of an alkalispray tank 20, alkali scrubber tank 21, warm water rinse tank 22, andhot air drier 23 and not using electrolytic washing and an alkaliscrubber using nylon brushes 24 are arranged. The reason why thegenerally used electrolytic washing is not used is that when using athin slab continuous casting machine and a hot rolling mill connectedwith it to produce hot rolled steel strip, the thin slab is hot rolled,then the hot rolled steel strip surface is pickled and coated with arust preventative. The time from the pickling to the hot dipping is 2days or less or so, therefore the amount of the rust preventative coatedmay be made smaller from the usual circumstances.

However, after pickling, the steel strip surface has a small amount ofrust preventative and rust preventative and Fe₃C etc. present on it, sothe alkali washing system not using electrolytic washing is used to washoff the rust preventative, Fe₃C, etc. adhering to the surface, then analkali scrubber using nylon brushes is used to remove the rustpreventative, Fe₃C, etc.

This washing removes the rust preventative usually burned off in aheating furnace. In the heating furnace, the oxygen in the atmosphere isused to stabilize the oxidation of the hot rolled steel strip surface.Therefore, the amount of formation of oxide film is stable, so this is agood condition for preventing nonplating defects.

Note that the suitable ratio of the amount of oxidation and amount ofreduction when dealing with hot rolled steel strip obtained by hotrolling a cast slab obtained by a thin slab casting machine was found byexperiments to be 0.4 to 0.55 or so. As opposed to this, in the case ofconventional cold rolled steel sheet, it was 0.2 to 1.2 or so, i.e., thevalues fluctuated.

Further, if using an oxidation process and reduction process like in thepresent invention, it was confirmed that even with hot rolled steelstrip produced by directly hot rolling a slab produced in a thin slabcontinuous casting machine and having a thickness of 2 mm or more, nocoil breakage occurs even if using the usual conveyance rolls of adiameter of 1500 mm in the processes after plating.

The reason is believed to be that by setting the temperature elevationrate at the oxidation process to 25° C./s and making the reduction timeshorter than that of the reduction process of conventional cold rolledsteel sheet, the yield point of the hot rolled steel strip becomeshigher and processing becomes possible at less than the strain whereyield elongation occurs, so coil breakage no longer occurs.

Note that the usual processing rate in the current art is 90 mpm to 180mpm, so it is possible to apply the present invention to newly establishor modify hot dipping facilities having this range of rates. The upperlimit of the processing rate of a hot dipping facility is, in thecurrent art, 180 mpm or so. However, even if a hot dipping facility withan even higher processing rate is developed, the present technology canbe applied. Further, the lower limit of the processing rate may be anyrate so long as the conditions of the present invention can be realized.

Some hot dip galvanization facilities are limited in terms of economicton/hr of the furnaces. In such a case, if the strip becomes thicker,the processing rate is reduced, so the time for passage through theoxidation furnace becomes longer and as a result the average temperatureelevation rate becomes smaller. In this case, the facility may also beoperated so that part of the temperature elevation process satisfies thetemperature elevation rate of the present invention.

EXAMPLE 1

The ingredients of four types of hot rolled steel strips A, B, C, and Dproduced using the thin slab continuous casting process are shown inTable 1 expressed by mass %.

TABLE 1 Steel C Si Mn P S Al N Ca type (mass %) (mass %) (mass %) (mass%) (mass %) (mass %) (mass %) (mass %) A SGHC 0.054 0.06 0.25 0.02 0.010.029 0.0081 0.0017 B SGHC 0.043 0.046 0.24 0.25 0.012 0.05 0.075 0.0023C SGH440 0.16 0.05 1.1 0.02 0.012 0.04 0.07 0.002 D SGH540 0.11 0.031.51 0.025 0.005 0.05 0.06 0.0025

The various conditions and results when using the method of productionof hot dip galvanized hot rolled steel strip according to the presentinvention to produce hot dip galvanized hot rolled steel strips fromthese four types of hot rolled steel strip are shown in Table 2. For theproduction of the hot dip galvanized hot rolled steel strips, four typesof hot rolled steel strips were passed through a preheating furnace,non-oxidizing furnace, reduction furnace, soaking furnace, and coolingfurnace for oxidation, reduction, and cooling, then were hot dipgalvanized.

The amount of coating of the hot dip galvanization was in the range of80 to 120 g/m² (one side).

TABLE 2 Peak max. Preheating (oxid) furnace + Overall Temp. steel Peakmax. non- Reduction Plating Hot Line oxidation elevation strip Reduction(reduction) oxidizing zone state G: Data rolled speed time rate temp.time steel strip furnace length good, P: no. steel (m/min) (sec) (°C./S) (° C.) (sec) temp. (° C.) length (m) (m) poor) 1 A 100 20 28 55039 710 33 52 G 2 B 120 19 29 560 36 730 38 62 G 3 C 100 20 29 570 39 75033 52 G 4 D 140 16 34 550 36 700 38 62 G 5 A 120 17 31 510 39 730 33 52P 6 B 120 19 32 600 36 750 38 52 P 7 C 77 26 21 550 50 710 33 52 P 8 D180 13 44 560 21 730 38 62 P 9 B 120 19 29 550 36 680 38 52 P

As shown in Table 2, the Data Nos. 1 to 4 are examples satisfying all ofthe conditions defined in the present invention. The surfaces of theproduced hot dip galvanized hot rolled steel strips were extremely goodin terms of plating state.

On the other hand, the Data Nos. 5 to 9 shown in Table 2 are comparativeexamples where some of the conditions defined in the present inventionare not satisfied. The surfaces of the produced hot dip galvanized hotrolled steel strips had nonplating defects or residual scale or otherplating defects.

EXAMPLE 2

The ingredients of two types of hot rolled steel strips A and B producedusing the thin slab continuous casting process are shown in Table 3expressed by mass %.

TABLE 3 Steel C Si Mn P S Al N Ca type (mass %) (mass %) (mass %) (mass%) (mass %) (mass %) (mass %) (mass %) A SGHC 0.054 0.06 0.25 0.02 0.010.029 0.0081 0.0017 B SGHC 0.043 0.046 0.24 0.25 0.012 0.05 0.075 0.0023

The various conditions and results when using the method of productionof hot dip galvanized hot rolled steel strip according to the presentinvention to produce hot dip galvanized hot rolled steel strips fromthese two types of hot rolled steel strip are shown in Table 4. For theproduction of the hot dip galvanized hot rolled steel strips, the twotypes of hot rolled steel strips were oxidized by a preheating furnaceand non-oxidizing furnace, reduced by a reduction zone (reductionfurnace and soaking furnace), then were hot dip galvanized. Note that inthis experiment, the preheating furnace and non-oxidizing furnacecorrespond to the “furnace used for oxidation”, while the reduction zonecorresponds to the “furnace used for reduction”.

TABLE 4 Peak Peak max. max. Preheating (oxid) (red) furnace + Ratio ofTemp. steel steel non- Reduction Plating length Hot Line Overallelevation strip Reduction strip oxidizing zone state G: used for Datarolled speed oxidation rate temp. time temp. furnace length good, P:oxidation/ no. steel (m/min) time (sec) (° C./S) (° C.) (sec) (° C.)length (m) (m) poor) reduction 1 A 100 20 28 550 39 710 33 53 G 0.63 2 B120 19 29 560 36 730 38 62 G 0.61 3 B 120 19 29 560 24 730 38 41 P 0.934 B 120 19 29 560 46 730 38 78 P 0.49

The Data No. 3 and 4 shown in Table 4 had lengths of preheating furnacesfixed at 17 m and lengths of non-oxidizing furnaces fixed at 21 m, haddifferent cooling conditions, and had lengths of reduction zonesadjusted to become pseudo 41 m and 78 m. The reduction time is the valuecalculated from a processing rate of 120 m/min.

As shown in Table 4, Data No. 1 and 2 are examples where the ratio ofthe total length of the preheating furnace and non-oxidizing furnace andthe length of the reduction zone satisfies the condition of being in therange of 0.5 to 0.9 defined in the present invention. The surfaces ofthe produced hot dip galvanized hot rolled steel strips were extremelygood in terms of plating state.

On the other hand, the Data No. 3 and 4 shown in Table 4 are comparativeexamples where the ratio of the total length of the preheating furnaceand non-oxidizing furnace and the length of the reduction zone isoutside of the range of 0.5 to 0.9 defined in the present invention. Thesurfaces of the produced hot dip galvanized hot rolled steel strips hadnonplating defects and other plating defects.

Note that, the present invention is worked in the range of processingrate shown in the examples. In this case, the upper limit of theprocessing rate is, with current technology, 180 mpm or so. However,even if a hot dipping facility with a further greater processing rate isbuilt, the present technology can be applied.

Further, the lower limit of the processing rate may be any rate so longas the conditions of the present invention can be realized. The usualprocessing rate in current technology is 90 mpm to 180 mpm, so some hotdip galvanization facilities are limited in terms of economic ton/hr ofthe furnaces. In such a case, if the hot rolled steal strip becomesthicker, the processing rate is reduced, so the time for passage throughthe oxidation furnace becomes longer and as a result the temperatureelevation rate becomes smaller. In this case, the facility may also beoperated so that part of the temperature elevation process satisfies thetemperature elevation rate of the present invention.

INDUSTRIAL APPLICABILITY

The present invention is effective for preventing nonplating defectsfrom occurring on plated surfaces when hot dip galvanizing hot rolledsteel strip produced by the thin slab continuous casting process.

1. A method of production of hot dipped hot rolled steel stripcharacterized by producing steel strip by casting by a thin slabcontinuous casting process and hot rolling, said steel strip containing,by mass %, C: 0.03% or more, Si: 0.02% or more, Mn: 0.15% or more, andCa: 0.001% or more, heating it to a peak maximum steel strip temperatureof 550° C. to less than 650° C. by a temperature elevation rate of 25°C./sec or more for 15 sec or more for oxidation, heating it to a peakmaximum steel strip temperature of 700° C. to 760° C. so that the timewhen the steel strip temperature is 570° C. or more is 25 sec to 45 secfor reduction, then hot dipping it.
 2. A method of production of hotdipped hot rolled steel strip as set forth in claim 1 characterized inthat said hot dipping is hot dip galvanization.
 3. A facility forproduction of hot dipped hot rolled steel strip which hot dips steelstrip produced by casting and by the thin slab continuous castingprocess and by hot rolling, which facility for production of hot dippedhot rolled steel strip is characterized by having a furnace used foroxidation and a furnace used for reduction and by a ratio of lengthbetween said furnace used for oxidation and said furnace used forreduction along a conveyance direction of said steel strip being 0.5 to0.9.
 4. A facility for production of hot dipped hot rolled steel stripas set forth in claim 3, characterized in that said steel strip passesthrough said furnace used for oxidation in a time of 15 sec to 25 sec.